Code communication system for railroads



June17, 1947. 2,422,519

CODE COMMUNICATION SYSTEM :FOR' RAIAELROADS Filed July 21, 1944 2Sheets-Sheet 1 FIGIZ.

3nt entot I Gttorneg B; mm 1 June 17, 1947'. w. M. BARKER 2,422,519

CODE COMMUNICATION SYSTEM FOR RAILROADS Filed July 21, 1944 2Sheets-Sheet 2 FIG.1B.

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Shift to Selector 0N ypical Selector Code J 0- Start 5e|ector CodeBrwentor Gttorneg Patented June 17, 1947 CODE COMMUNICATION SYSTEM FORRAILROADS William M. Barker, Greece, N. Y., assignor to General RailwaySignal Company, Rochester, N. Y.

Application July 21, 1944, Serial No. 545,973

7 Claims.

This invention relates to communication systems for railroads, and moreparticularly to a code communication system of the so-called shunt typefor centralized traffic control systems combined on the same pair ofline wires with a telephone system using electro-mechanical selectors atway stations to effect selective calling of way stations.

In one type of code communication system for centralized traflic controlon railroads, commonly known as the shunt type system and disclosed, forexample, in the prior application of Hailes, et al. No. 484,728, filedApril 27, 1943, now U. S. Letters Patent 2,399,734 granted May 7, 1946,the code elements for controls and indications are transmitted over apair of line wires without changing their continuity and in such a waythat these same line wires may be employed for telephone communicationand the like.

In a code communication system of this character, conveniently termed aC. T. C. system, the field start to initiate operation of the system isaccomplished by shunting the line wires at a field station where achangein the occupancy of a track section or the like calls for thetransmission of new indications. A field station equipment is operableto shunt the line wires for such a field start only when the associatedline relay is energized with a certain polarity.

Assuming such a type of code C. T. C. system, it is often desirable toemploy the same line wires for telephone communication and selectiveringing by electro-mechanical selectors bridged across the line. Onetype of selector system, commonly used, and known as the WesternElectric train dispatching telephone system, uses an electromechanicalselector of the type which is operated by pulses of alternate polarity,and which is sometimes called an A. C. selector.

When such a selector system is combined with a code C. T. C. system ofthe shunt type above mentioned, it may happen that some of the selectorcode pulses will be of sufficient intensity and duration to energize theline relays of the v C. T. C. field equipment with the polaritypermitting a field start, thereby allowing a C. T. C. field stationwhere some change in track circuit occupany or the like has occurred, toapply a shunt across the line wires during transmission of the selectorcode and thereby mutilate that code, or otherwise interfere with thedesired operation of the combined system.

In view of these considerations, the primary object of the presentinvention is to provide an improved organization and arrangement ofparts and circuits for a centralized traflic control system and amechanical selector system combined on the same pair of line wires, insuch a way that the line relays of the C. T- C. system are not operatedby the pulses in the selector code, or at least are not energizedsufliciently to permit a field start.

Generally speaking, and without attempting to define the nature andscope of the invention, it is proposed to combine the batteries or likesources of current for the code C. T. C. system and the mechanicalselector system in such a way that the desired changes in line circuitenergization required for satisfactory operation of theelectro-mechanical selectors are produced without creating a differenceof potential between the line wires of the proper polarity and intensityto operate the biased polar line relays of the C. .T. C. system andpermit any field start to shunt the line circuit during a selector code.Specifically, it is proposed, when sending selector codes, to connectthe C. T. C. line battery in series with the line battery of theselector system with the polarity sense that will permit reversing theselector battery, in order to produce the alternating current componentsrequired to operate the selectors, without permitting the voltagebetween the line wires to reach the operating value of the biased polarline relays of the C. T, C. system.

Other objects, characteristic features, and attributes of the inventionwill be in part apparent, and in part pointed out, as the descriptionprogresses.

The accompanying drawings illustrate in a simplified and diagrammaticmanner one specific embodiment of the invention applied to a shunt typeC. T. C. code communication system, such as disclosed in theafore-mentioned Hailes et a1. Patent 2,399,734. These drawingsillustrate certain relays and contacts of the complete system disclosedin this Hailes et a1. application, insofar as appears to be necessaryfor an explanation and understanding of the present invention; and forconvenience these relays and contacts are given the same referencecharacters as in the Hailes et a1. application.

In the accompanying drawings:

Figs, 1A and 1B show the equipment at the control oftlce and at onetypical field location respectively;

Fig. 2is 'a diagram of a simplified equivalent circuit to' illustratethe way in which the batteries are combined during transmission of aselector code; and

Fig. 3 is an explanatory chart or diagram illustrating the characterofenergization of the line wires for a typical selector code.

Considering first the C. T. C. code communication system, it is notnecessary to describe the structure and operation for the completesystem, but merely point out the features or operating characters of thesystem material to an understanding of the present invention. Thisrefers more particularly to the conditions under which the fieldstations act to shunt the line wires for an automatic field start.

Under the normal or at-rest condition shown, the line wires LI and L2are steadily energized with one polarity, conveniently termed a normalpolarity, from the control ofiice battery CB, or equivalent source ofcurrent, by a. circuit which may be traced from the line wire Ll,through back contact 5 of a shift or transfer relay SI-I, through coils2T, 26 of the filter, back contact 25 of relay CF, front contact 24 ofrelay OR, back contact 23 of relay C, ballast lamp BAL to one terminalof the battery CB, assumed to be through primary 29 of the impulsetransformer 30, back contacts 3-! and 32 of relays C and CF, coils 33,34of the filter, and back contact B of relay SH to the other line WireL2.

Referring to Fig. 13, this energization of the line circuit with thenormal polarity causes current to flow through the line relay-2F at thetypical field station illustrated, and similar line relays at all theother field stations, over a circuit path which may be traced from theline Wire Ll through coils 3E, 36 of a filter, back contact 3'! of relay2L0, upper winding of relay 2F, resistance 38,1ower winding of relay 2F,back contact it of relay 2L0 and coils M, -42 of the filter to the otherline wire L2. This normal polarity energizes line relay 21? to cause itspolar contact 213 to assume the right-hand positionshown in oppositionto its spring bias and close a pick-up circuit for a change repeaterrelay -2CI-IP which may be traced from (+),'through polar contact 214 ofline relay 2F, front contact-332 of relay ZFR, backcontact 833 of relayZLOT, back 'con- 5 l tact 3-34 of relay 2CD, front contact 335 of relayzF Ryrelay ZC-HP, and through back contact'3'36 of' ch-a nge relay 2Cl-Ito The change relay ZCHisnormallymaintained energized 'by a stickcircuit including-contacts operated to momentarily break such stickcircuit'upon operation of a track relay,,signal, switch position"indicating relay, or like device having its position indicated in thecontrol office. The contacts f45 and 46 shown as operated by trackrelaysATR and TR are typical of such contacts included in the stick circuitfor the relay ZCI-I readily traced from through saidcontacts 4'5, '46,front contact '4? of relay 20H, winding ofthis relay, to

Thus, under the normal or at-re'st condition shown, if the track relayTR, for example, should pickup or drop due to a change in the occupancyof its associated ,track section, the stick circuit for the change relayZCH is momentarily broken to release this relay and close itsbackcontact 336, thereby permitting ,energization of the change repeaterrelay 2CHP by the pick-up circuit above pointed out. The energizationofthe relay 2CHP and closure of its front contact 3 42 closes a pick-upcircuit to restore the'change' relay :ZCH and render it eifective torespond to any subsequent changes in the condition of'the devices to beindicated, for the purposef'and in the manner more fully explained inthe I-Ia'ileset al.

4 application above mentioned. The energization of the relay ZCHP andclosure of its front contacts 337 and G2! provides stick circuits formaintaining this relay energized until the field station in question hasoperated to transmit its indications, one of these stick circuits beingreadily traced from through said front contact 421, back contact $26 ofa step'relay V5 in multiple with a back contact of relay 2L0, frontcontact 335 of relay ZFR, winding of relay ZCI-IP, and its front contact331 to Another stick circuit includes back contacts of relay 2L0 and afront contact of relay ZLOT.

Also,- the energization of the change repeater relay 2CI-IP in themanner just described and closure of its front contact 340 provides acircuit for energizing the relay 2E0 to close its front contact 35d,this circuit being traced from through front contact 338 of relay ZFR,back contact 339 of timing relay ZLE-T, front contact 3% of relay ECHRback contact 236 of relay winding of relay 2E0 to The-energization ofrelay EEG and closure of its front contact'352 completes an energizingcircuit for the relay {2L0 including various other contacts indicated bycrosses and readily traced on the drawing.

This energization of the relays zEQ an-d 2L0 at a field station directlyconnects the line Wire L1 to line wire L2 through the filter by acircuit connection through front contact Ml of relay 210, front contact1354 'of relay EEG, and back contact355 of relay ZEE.

If the line relay at a field stationgsuchas relay 2F in Fig. 1B,is'ueenerg-ized, or isenergized with a reverse polarity. its contact2-14 is in'the left-hand dotted linepositi'on, and'the pick-up circuitfor the change repeater relay ZCH-P is open jand regardless of anydeenergizati-on of the change relay "20H, the field stationcannot-operate to shunt the line circuit for a field start, so long assuch condition ofthe line-relay 1F exists. Similarly, if thereare's-tored field starts, i."e., a CH? relay ispicked up at any fieldlocationsysb long as the C. T.--C. line relays such as relay "2F aredeenerg ized, or energized with a'reverse'polar-ity, no C. fieldlocation can pick up its E0 and LOrelays tosh-unt the linecircu'it.

In short, in the type 'of codeC. T. C. system under consideration, anautomatic field start from one or more'field stations'to apply a shuntto -,the line circuit cannot occur unless the line relays-at these fieldstations are energizewwith thenormal or at-rest-polarity;,and thisinvention is concerned m'ore particularly with the problem of preventingerrergization of the line relays of the field stations with such normalat-restpolarity during the transmission of a selector "code involvingimpulses of alternate polarity, I

Considering now the selector "system, a shift or transfer relay Sl-I, whi'chi'sarran'ged 'tobe en'- ergized upon manual actuation by theoperator of, a suitable key orswitch 'S, subject-to such interlock asdesired and indicated generally by the back contact 9 of relay C, actsto'disconne'ct the control office line unit and stepper .from the lineWires L1 and. L2, and 'to connect the'se'lin'e Wire's through the frontcontacts 5 and "6 of the relay 'SI-Ito the apparatus case o'ftheselector system. In short, manual 'energizati'on of the relay'jSHshiftsfthe line-wire connections from the 'C.'T.TC. system to theselector system. V

It is assumedthat'the, selector system is the well-knownWesternEl'ectric train dispatching telephone system employing selector'and calling key structures such as disclosed'for example, in

the patent to J. C. Field, No. 1,343,256, June 15, 1920. The controlofiice equipment for this selector system comprises a pulsing relay PLoperating spring biased contact fingers 10, H for pole-changing theconnections between a selector battery SB, or equivalent source ofuni-directional current, with respect to the input terminals of atypical filter l 2. A connector relay CR operates biased contact fingersl4, [5 to connect the output of this filter [Z to the Wires ll, l8 whichare connected to the line wires LI and L2 through the front contacts 5and B of the relay SH when that relay is energized.

The connector relay CR is energized at the beginning of each selectorcode and is maintained energized throughout transmission of such code,while the pulsing relay PL is intermittently energized at time spacedintervals in accordance with the particular selected code being sent bythe operation of an individual calling key or equivalent selector keystructure. The code wheel CW for one such individual calling key isshown diagrammatically in the drawing as typical of such controldevices; and after the calling key has been wound up and released in theregular way, the code wheel CW is turned in the counter-clockwisedirection indicated by the arrow by a return spring at a regulated rateto apply to the line circuit a fixed total number of pulses, e. g. 17,divided into three groups of varying number, such as 3l0-4, l-9 l, etc.

The selector for this system under consideration, shown diagrammaticallyin Fig. 113, has an operating winding i9 connected across the line wiresLI and L2 in series with a condenser 20 of suitable capacity. Thisoperating winding l9, when energized with current pulses of oppositepolarity, operates a polarized armature 2| back and forth from andthrough a middle biased position. Each movement of this armature 2| fromits middle position, or a movem nt quickly from one extreme position tothe other, operates a pawl and ratchet structure to advance a steppingwheel and contact arm one step; but when the armature pauses in themiddle position, as distinguished from a quick movement through thatposition, the stepping wheel is returned by a spring to its initialposition unless a pin is provided to hold said wheel in that position.The location of these pins determines the code call for the selector.Various other operating characteristics of this type of selector areshown and described in detail in the J. C. Field patent above mentioned.

Considering a typical operation, assume that the operator desires tosend a selector code and operates the switch to energize the shift relaySH at a time when the C. T. C. system is not in operation, and it isotherwise proper to pick up this relay SH and transfer from the C. T. C.system to the selector system. This deenergizes the line wires Ll, L2and all line relays such as 2F at all of the field stations, therebyrendering all of these stations ineifective to shunt the line for afield start. The operator then actuates the individual calling key forthe particular selector station desired, or performs a similar functionby manipulation of other selector key structures.

Assuming the code wheel CW shown is thus operated, the initial movementof this code wheel closes a circuit to energize the connector relay CR,which may be traced from contact finger 50, code wheel CW, slip ringconnection 5|, and wire 52 through relay CR to This energization of theconnector relay CR acts to connect the C. T. C. battery CB'and theselector battery SB in series to the line wires LI and L2 with thereverse polarity by a circuit connection which may be traced from linewire Li, front contact 5 of relay SI-I, wire ll, contact 14 of relay CR,telephone filter l2, wire 55, battery CB, upper or terminal of batteryCB, wire 56, contact iii of relay PL, through battery SB, contact II ofrelay PL, filter l2, contact l5 of relay CR, Wire l8, and front contact6 of relay SH to the other line wire L2.

It will be noted that the batteries CB and SB are connected to actaccumulatively, as indicated in Fig. 2, so that the potential across theline wires LI and L2 is the sum of the voltages of these two batteries.Generally speaking, the impedances and number of the connections for C.T. C. field stations and selectors are much alike, so that ordinarilythe voltages of the two batteries CB and SB for operating the C. T. C.field stations and the selectors respectively are much the same; and forpurposes of explanation, it is assumed that these batteries are the sameand each is a volts.

It should also be noted that when the connector relay CR for theselector system is energized to provide the first energization oftheline circuit, the polarity of this energization is opposite to thenormal at-rest polarity, so that contact fingers of all of the linerelays, such as the contact finger 214 of line relay 2F, remain in thereverse or dotted line position, which they assumed when relay SH pickedup and deenergized line Wires LI and L2, thereby maintaining open thecircuit for providing a field start. Considering this reversal ofpolarity more in detail, upon referring to Fig. 1A, it will be notedthat the line wire Ll is connected to the terminal of the battery CBunder the conditions shown with the relay SH deenergized; but if thisrelay SH is energized, then the same line wire Ll is connected throughfront contact 5 of relay SH, wire ll, contact M of relay CR, filter l2,and wire 55 to the terminal of the battery CB.

The initial energization of the line circuit at the beginning of aselector code as just described, sends current through the operatingwinding 19 of all of the selectors during the charging time of theassociated condenser 20, so that the armature of each selector is movedto one extreme position and then, after the condenser 20 becomes fullycharged, assumes its middle position. This serves to restore to theinitial position the stepping Wheel for any selector that may happen tobe in an intermediate position.

Referring to the explanatory diagram in Fig. 3, this clear-out pulse,which is caused by the energization of the line circuit by the twobatteries CB and SB acting cumulatively represents a line circuitenergization of 200 volts on the assumption previously mentioned thateach of these batteries is 100 volts.

After this preliminary movement of the code wheel CW, the first tooth ofthis wheel lifts the arm 50 into engagement with a stationary contact 58to close a circuit readily traced on the drawing for energization of thepulsing relay PL, thereby operating its contacts H], I l to poleohangethe connections and in eifect connect the batteries CB and SB inopposition. Assuming these batteries to be equal, the impressedpotential across the line wires LI and L2 drops to zero, as indicated inFig. 3, for the first counting pulse designated I, 'When the first toothpasses from arsenic under the contact arm 50 and the relay'PL isdeenergiz'ed, the line wires LI, L2 are again energized for count 2 witha potential of 200; and the next time the contact arm 50' is raised bythe code wheel CW for count 3 of the selector code, the potential acrossthe line wires LI and L2 again falls to zero.

In the case of the code 3-10-4' assumed, the line circuit conditionafter the third counting pulse is maintained until the condenser 29assumes a stable condition and no more current flows through theoperating winding ['9' of the selector, whereupon its armatureassumesthe middle position, In accordance with the wellknown plan ofoperation for this type of selector system, the stepping wheels of thoseselectors having a code call beginning with a count of 3 are held by apin while their armatures are in the middle position, but all of theother selectors having a different code are restored to the initialposition. This same operation is repeated for two more groups of pulsesto make up a predetermined code count of 17 in the particulararrangement assumed; and that at the end of the transmission of these 17pulses in three groups of a certain number in each group, such as3-l0--4, there is only one selector having this particular code callwhich has had its stepping wheel advanced the full number of steps toits final position; and hence the last pulse is effective at this oneselector to close a circuit for ringing the hell or perform any otherdesired function. After the last or bell ringing impulse, the code wheelis shaped to cause energization of the connector relay CR for a timewhile the pulsing relay PL-is deenergized. This restores the selector inquestionto its initial condition; and when the code wheel assumes itsinitial position shown, the relay CR. is released.

From the standpoint of the operation of the selector, which is connectedacross the line wires LI and L2 through a condenser 20 of the propercapacity, these changes in line circuit energization between zero and200 indicated in Fig. 3,. are in effect alternating components. It maybe said that each time the line circuit is energized, the condenser 20is charged and the operating magnet !9 of the selector is energized withcurrent of 7 one polarity; and each time the line circuit isdeenergized, the discharge of the condenser 29 causes a flow of currentthrough the operating windingof the selector in the opposite direction.The net result is that the selector operates the same as if 100 voltimpulses of alternate polarity were impressed across the line wires LIand. L2.

Stated another Way, in accordance with the Well-known transpositiontheorem, the inclusion of the C. T; C. battery GB in the circuit forenergizing the line wires. Li and L2 for selector code transmission doesnot alter the current conditions produced by the selector battery SB"alone, except with respect to the impedance that the C. T, C. battery CBmay add to the circuit.

Referring particularly to a-diagram of: Fig. 3, it can be seen thatduring the transmission of this typicalselector code, the voltage of theline wire Ll with respect to L2 varies-from 200 to zero, but the lineWire Ll never becomes positive with respect to the line wire L2.Consequently, current through the line relays ofthe C. T. C. fieldstations, such as the relay ZF in Fig. 1B varies from a reverse polarityto zero. The armature of the line relay: 2F is biased by a spring, asindicated conventionally, to assume the same left-handp'osition whendeenergized as it assumes when ener'gized withthe reverse polarity. Inshort, during selector cod-e transmission the armature of the line relay2F remains inthe left-hand position, being held there by the biasingspring alone, or by the biasing spring plus electr'o-magneticattraction.

In this way, the line relays of the C. T. C. field stations areprevented from assuminga condition, even momentarily, for a field start;and

' hence no stored start or change in track circuit occupancy or the liketending to create a field start can become efiective while the system isset for transmission of selector codes, thereby obviating any mutilationof the selector codes by a shunting of the line circuit for a fieldstart during the transmission of such selector codes.

In this connection,- it should be appreciated: that it is not necessarythat the C; T. C. battery CB and selector battery SB should exactlyequal each other and provide a zero" potential for the line circuit.This condition has been assumed to simplify an explanation andunderstanding of the nature of the invention,'but i-t can be seen thatany relation hip of battery voltages may be employed, so long as thereis not enough 'difierence of potential between the line wires of thenormal at-rest polarity to provide pick-up energy for the C. T. C. linerelays. Such pick-up energy is ordinarily in the order of 50 volts, andthe voltage for the selector battery SB may exceed that of the C. T. C.battery CB and cause a POSie tive energization of the line wire Ll withrespect to L2, provided such positive energization is not of the C. T.C. battery CB, but if desired, the,

wires 55, 56 may be connected so as to use the appropriate portion ofthis battery; or to include extra cells if the selector battery shouldhappen to be substantially greater than the: C. T. C.

battery. V r

The particular construction and arrangement of parts shown and describedmerely represents one typical specific embodiment of the invention; andI desire to have'it understood that various modifications; adaptationsand additions may be.

made in this particular construction and arrangernent of parts withoutdeparting fromth invention.

What I claim is: 1. In a system of centralized trafiic controlcombinedon the same line wires with a selector system, a plurality ofselectors. bridged. across said line wires with coupling condensers, aplurality of field station equipments each including a polar line relayconnected across said. line wires and operable only when said line relayis energized with a particular polarity to permit that field stationequipment to shunt the line wires for creating an automatic field start,calling means in a control oflice including a pole-changed selectorbattery for changing the energization of said line wires to" operatesaidzselectors, and manually operable means for rendering said calling 7means effective and for also connecting another" source of currentto-the-line circuit in series'with said selector battery a'id anothersource of -c1ir= rent and said pole-change selector battery actingcumulatively and in opposition alternately to provide pulses on saidline wires of a polarity opposite said one particular polarity and at anenergy level suitable for operating said selectors.

2. In a system of the character described for combining a centralizedtraffic control code system and a selector code system on the same linewires, code transmitting means at a plurality of field stations eachincluding a polar line relay and operable only if said line relay iseifectively energized with a particular polarity to shunt said linewires for an automatic field start, code transmitting means in thecontrol oflice including a source of direct current for normallyenergizing said line Wires with said particular polarity and operable inresponse to said automatic field start to energize said line wires withthe other polarity, mechanical selectors connecting across said linewires with coupling condensers, a selector battery in a control ofiice,calling meansin said control ofiice for intermittently pole-changing theconnections between said selector battery and said line wires inaccordance with a selector code, and manually operabl means forrendering said calling means effective and for also connecting saidsource of direct current in series with said selector battery to provideintermittent energization of said line wires with a polarity differentfrom said particular polarity during the po1e-changing of the selectorbattery for transmission of a selector code.

3. A combined centralized trafiic control and mechanical selector systemcomprising, a pair of line wires connecting a control ofiice and aplurality of field locations, code transmitting means at certain of saidfield locations each including a polar line relay normally energizedwith one polarity, mechanical selectors at the other field locationsconnected across said line wires by coupling condensers, calling meansin the control ofiice including a selector battery and a pulsing relayfor pole-changing the connection between said selector battery and saidline wires in accordance with a desired selector code, code transmittingand receiving means at the control office including a separate batterynormally energizing said line wires with said one polarity, and manuallycontrollable means for rendering said calling means effective and forconnecting said separate battery across the line Wires with its polarityreversed in series with said selector battery and independently of saidcode transmitting and receiving means, whereby a potential between saidline wires is maintained independently of the pole-changing of theselector battery to prevent effective energization of said line relayswith said one polarity during transmission of a selector code.

4. In a centralized traffic control and selector system combined on thesame pair of line wires, a plurality of field station equipments eachincluding a polar line relay connected across said line wires, each ofsaid field station equipments being operable only if its polar linerelay is energized with a particular polarity to create a field startcondition by shunting said line wires, code transmitting and receivingmeans in a control ofiice including a coding source of current fortransmitting control codes to selected field station equipments and forinitiating transmission of indication codes from a selected fieldstation equipment in response to a field start condition, selectorsconnected across said line wires at field locations, calling means inthe control office including a selector battery and a pulsing relay forpole-changing the connections between said battery and the line wires inaccordance with a selector code, and manually controllable means in thecontrol olfice forrendering said calling means effective and said codetransmitting and receivingmeans ineffective and for also connecting saidcoding source of current with its'polarity reversed with respect to theline wires in series with said selector battery, whereby duringthetransmission of a selector code the line wires do not assume suchdifference of potential required for the effective energization of saidline relays with said particular polarity'to permit a field start.

5. In a code communication system for railroads, a pair of line wiresconnecting a control ofiice and a plurality of field stations, codetransmitting means at each field station including a polar line relayand operable only when said line relay is energized with a particularpolarity to govern the energization of said line wires for transmissionof a field start condition to the control ofiice, a mechanical selectorbridged across said line wires with a coupling condenser and responsiveto the number of changes in the energization of said line wires during aselector code, said selector requiring for its operation changes in thelevel of energization of said line wiresequivalent to pulses of acertain voltage and alternate polarities, coding equipment at thecentral office normally energizing said line wires with a predeterminedvoltage of said particular polarity to maintain said line relaysenergized to permit an, automatic field start, and manually operablemeans in the control ofiice including a selector calling key forintermittently energizing said line wires with pulses of the samepolarity opposite to said particular polarity in accordance with aselector code, said intermittent energization of said line wires beingat a voltage higher than said predetermined voltage and substantiallydoubl said certain voltage required for selector operation by alternatepolarities, said higher voltage acting by charging and discharging saidcoupling condenser at a high energy level to cause operation of saidselector substantially the same as in response to pulses of said certainvoltage and of alternate polarity.

6. In a code communication system of the character described forrailroads, a pair of line wires connecting a control office and a numberof field stations, a plurality of mechanical selectors bridged acrosssaid lino wires with coupling condensers and responsive to thenergization of said line wires, coding equipment at each field stationconnected to said line wires and including a polar relay governing theoperation of said equipment, said coding equipment including meansefiective only if said line relay is energized with one particularpolarity to apply a shunt to said line wires for a field start,transmitting means at the control office including a source of directcurrent for energizing said line wires with said one particular polarityfor the normal at-rest condition of the system, an additional source ofdirect current in the control office, and manually operable means in thecontrol ofiice including selector calling keys for intermittentlyenergizing said line wires from both of said sources of current actingcumulatively and in opposition alternately to form pulses effective foroperating said mechanical selectors and all having the same polarityopposite to said one particular polarity, whereby the condenser couplingthe mechanical selector to the line wires is charged and discharged atan energy level by the combined sources of current to cause 11 eeiraibion .ef the .selechor'snbstantially the .same as inresponse topulses of ai'ternatve pplarity irpm saidwadditienail .source alone.

'7; In a communicafion system 9f the chamoier described, a. pair ofiinawiresiconnecitfrri a .6011:- tizol omce and a, plurality mf' fieldstations; pads transmittin means atzeaich-iield station includ a, palmiine relay governingin oneieneit ized' positn'en only the autamazticfield smart gontxoi: from $hait'fie1d; station, a-pluxalityntmeshariicai seleetors' bridged across said line wires with muplincondensers and m rated by cm-mass firin wire energization, sodmg meansin the 5011111191 office including a iselecmr mum: of ciurxem and codegenerating devices for pplg changixlg. the electrical iepnnectionsbetwaen mid, 5.81%?292. mums of aurrent'andsaid line-wims-in accswrdamthe .desinedselecter vcsxele fer @elmiflue epemtienm saidseieemrs',.andrmearis .ferirem'ler sng .S L -fi Imimeans efiecnive and for aonnestng; answer scmmeni 0111mm; 410 said mewiresdn :sexi'eswith said se1et r.spm'cg 10.5 sunrent' to pm -eilt. gnexgi ation 9i said line wipes witha polarity to energize said line relays ito rsiaikd one engr zeslposition, the navel of ener izaiion of aid .liane wires by combininisaid sou ces Qf current bein chan ed. by -p.o1e-chan ng said sel c orbattery to cause operation of ,said sa ector;

WILLIAM BARKER.

CITED The following referen es are f record in the file 79f this patent:

UNITED STATES PATENTS

